Any serious web developer has had to deal with determining page load time. No other issue is as important as how fast a page loads. You can have an amazing site, but if it takes too long to load, your users (or customers) will not wait around for your site to load.
One good diagnostic tool to help measure page load time is jQTester. It is a plugin that has you place small amounts of code at the top and bottom of your page. When the page finishes loading, you get a notification saying how long it took the page to load.
This is incredibly helpful to have in your web development toolbox.